When a base station of a wireless communication system transmits a multicarrier signal that includes signals at multiple different carrier frequencies, passive inter modulation distortion (hereafter, referred to as the “PIM” as appropriate) sometimes occurs in the signal that is received from a terminal. The PIM is one of the types of inter modulation distortion (hereafter, referred to as the “IM” as appropriate). For example, if the multicarrier signal, including the signal at a carrier frequency f1 and the signal at a carrier frequency f2, is transmitted, the signal that is received from the terminal sometimes includes third-order PIM. Hereafter, the signal at the carrier frequency f1 is referred to as the “carrier signal 1”, and the signal at the carrier frequency f2 as the “carrier signal 2”. In the case of transmission of the multicarrier signal that includes the carrier signal 1 and the carrier signal 2, third-order PIM occurs at the frequency of “2f1−f2” and the frequency of “2f2−f1” on the frequency axis.
The PIM interferes with the received signal; therefore, if the PIM occurs in the received signal, the signal-to-interference power ratio (SIR), which is one of the reception qualities of the base station, deteriorates. The base station uses the SIR to control the transmission power of the terminal. Therefore, it is important for the base station to know the level of the PIM with accuracy.
Furthermore, as the technique for knowing the level of the IM, there is a disclosed technique that uses signals at multiple carrier frequencies, included in the multicarrier signal, to generate a replica signal for the IM and detects the correlation value between the replica signal for the IM and the received signal as the level of the IM.
Japanese Laid-open Patent Publication No. 2014-57327
However, in the conventional technology, no consideration is given to prevention of degradation of the reception quality due to the passive inter modulation distortion.
Specifically, according to the conventional technology, the correlation value between the replica signal for the IM and the received signal is simply detected; therefore, if the correlation value increases and large IM occurs in the received signal, the SIR, which is one of the reception qualities of the base station, is degraded. Similarly, if the PIM, which is one of the types of IM, occurs in the received signal due to the transmission of a multicarrier signal, the SIR deteriorates. Degradation of the SIR is undesirable as it decreases the accuracy with which the base station controls the transmission power of the terminal and, as a result, it causes an increase in the power consumption of the terminal.
Therefore, it is expected to prevent degradation of the reception quality due to the passive inter modulation distortion.